First Two Layers, or F2L are normally the first two bottom layers of the 3x3x3 cube, or essentially all layers up until the last layer on larger cubes.

The definition is a little different depending on the subject or who you are talking to. Usually, it refers to the part of the CFOP and CFCE methods that solve the first two layers in pairs, excluding the cross step.

2-gen F2L, most pairs (30 of 41) have the edge sitting in the U-layer having two orientations, it shows R or F colour at the side. If it is the R colour you can solve the pair using only RU and if it is the F colour you can turn the cube (y') and sovle the pair to the back slot using only RU. The only pairs that needs both R and F turns are the six ones with the edge unoriented in the slot, if the edge is oriented in the slot both ways works.

Approaches

Petrus F2L

Another way to solve the 'F2L' is by building blocks, common during the first two layers of the Petrus method.

Fridrich F2L

There are many ways to solve the 'F2L' on a cube. A common system is using the Fridrich method first two layer approach. After solving the cross, a corner-edge pair is paired up (form pair), and then inserted into the correct slot (pair insertion). A total of four corner edge (or 'CE') pairs are made and inserted to solve the first two layers. This step of the cube should be done intuitively, even though there are algorithms for each case, algorithms should only be used for slower cases.

The concept of pairing up four corner/edge pairs was first proposed by René Schoof in 1981.